In Web API, there's currently no way to create a link to a path under the app's virtual root. The solution is probably something like:
UrlHelper.Content("~/<path>");
Comments: commit dbcacb440b035ff643c2afe8dff68015ea8cdf3e Author: Ryan Nowak <nowakra@gmail.com> Date: Wed Aug 7 19:45:55 2013 -0700 Codeplex-1082 - Add a Content method for generating virtual-path based urls
UrlHelper.Content("~/<path>");
Comments: commit dbcacb440b035ff643c2afe8dff68015ea8cdf3e Author: Ryan Nowak <nowakra@gmail.com> Date: Wed Aug 7 19:45:55 2013 -0700 Codeplex-1082 - Add a Content method for generating virtual-path based urls